1. Field of the Invention
The present invention relates to a mobile phone or other wireless communication terminal able to utilize a plurality of wireless networks and to an antenna switching method. In more detail, the present invention relates to a wireless communication terminal able to communicate by a transmission system without use of diversity (for example CDMA2000 1x) and a transmission system with use of diversity (for example CDMA2000 1xEVDO) and having receiver circuits with frequency bands where these frequency bands interfere with each other and to an antenna switching control method for the same.
2. Description of the Related Art
In recent years, mobile phones and other wireless communication terminals able to utilize a plurality of wireless networks have come into use. These types of wireless communication terminals are configured so that communication quality information such as communication speed can be displayed.
Further, as a high speed wireless communication system, the CDMA2000 1xEVDO (EVolution Data Only) system is now coming into use.
The communication quality information of the currently used CDMA2000 1x transmission system does not change much in data transfer speed according to location, therefore the state of reception etc. is determined on the basis of an instantaneous value such as the ratio of the pilot signal strength to the total reception signal strength (Ec/Io) found from the pilot signal received from a base station and a carrier-to-interference ratio (CIR).
On the other hand, the communication quality information of the CDMA2000 1xEVDO transmission system is the average of the data transfer speed (DRC: Data Rate Control Bit) predicted on the wireless communication terminal side on a basis of the reception state (received field strength, C/I (Carrier to Interference), etc.). This is displayed on a display portion and notified to the user (see for example Japanese Unexamined Patent Publication (Kokai) No. 2002-369247 and Japanese Unexamined Patent Publication (Kokai) No. 2002-344560).
Due to this, the user of a wireless communication terminal employing the CDMA2000 1xEVDO system can obtain a correct grasp of the communication state.
A wireless communication terminal employing this CDMA2000 1xEVDO system assumes use under a strong field. The antenna gain is therefore not viewed as that important.
In order to improve the reception performance, the wireless communication terminal is configured so as to communicate by a diversity method using a plurality of antennas and receiver circuits so that the performance is obtained even under a fading environment. A mobile device can be configured even by one receiver circuit, but a better performance is obtained by two or more receiver circuits.
In a wireless communication terminal employing the CDMA2000 1xEVDO system, specifically an RF front end is configured by a transmitter/receiver circuit for the main antenna and a dedicated receiver circuit for the sub antenna in combination.
The signals obtained from the plurality of receiver circuits are combined by the maximum ratio combining method or the minimum mean square error method to compensate for deterioration of the reception performance in a fading environment.
In general, comparing the reception state in a case of using one antenna (not using a diversity method) and the reception state in a case of using two antennas (using a diversity method), it is learned that the influence due to interference is extremely small in the case of using two antennas (using a diversity method) compared with the case of using one antenna (not using a diversity method).
On the other hand, a wireless communication terminal used in the CDMA2000 1x transmission system assumes wide area service and desirably can be used in an environment with a weak field. Further, it is configured by a single receiver circuit. Accordingly, one antenna, that is, the main antenna, is employed.
If simply configuring a dual mode terminal compatible with both the CDMA2000 1xEVDO system and CDMA2000 1x system along with the spread of use of the CDMA2000 1xEVDO system, there are the following disadvantages.
As explained above, the properties of a terminal required under the CDMA2000 1xEVDO system and CDMA2000 1x system differ. Further, the frequency bands used in the two systems are the same or close.
Accordingly, if designing a dual mode terminal compatible with both systems, there is possibility that the performance will end up being inferior to that of a terminal designed for each individual system.
For example, if making a terminal of the CDMA2000 1xEVDO system compatible with the CDMA2000 1x system, the problem of interference between the main antenna and the sub antenna will lower the antenna performance. As a result, the area in which the terminal can be used in the CDMA2000 1x mode will become narrower.